\documentclass{article}\begin{document}
\begin{theorem}

\declare{syntax={infix,108,"+",l}, meaning={#_1 + #_2}}
\declare{macro=\add, argspec=[2][blablabla], meaning={#_1 + #_2}, meaningOpt={7}}
$\add{9}{2}$
$\add{1}$
\end{theorem}
\end{document}

